Lithium battery cathode composite material

ABSTRACT

A lithium battery cathode composite material includes a number of composite particles. Each of the composite particles includes one lithium vanadium phosphate particle and a lithium iron phosphate layer. The lithium iron phosphate layer is disposed on a surface of the lithium vanadium phosphate particle. The lithium iron phosphate layer includes a number of uniformly disposed lithium iron phosphate particles.

RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Applications No. 201010191050.4, filed on Jun. 3, 2010; No.201010191051.9, filed on Jun. 3, 2010; 201010191130.X, filed on Jun. 3,2010; and 201010191251.4, filed on Jun. 3, 2010, in the ChinaIntellectual Property Office, the contents of which are herebyincorporated by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to cathode material of lithium batteriesand methods for fabricating the same.

2. Description of Related Art

Lithium batteries are used in various portable devices, such as notebookPCs, mobile phones, and digital cameras because of their small weight,high discharge voltage, long cyclic life, and high energy densitycompared with conventional lead storage batteries, nickel-cadmiumbatteries, nickel-hydrogen batteries, and nickel-zinc batteries.

Among various cathode materials, transition metal oxides and mixedtransition metal oxides have received much attention because of theirrelatively high charge/discharge capacities in the lithium batteries.Lithium iron phosphate (e.g. LiFePO₄), and lithium vanadium phosphate(e.g. Li₃V₂(PO₄)₃) are two widely used cathode active materials. Lithiumiron phosphate has the advantage of high specific capacity, but has thedisadvantage of bad performance at low temperatures. Lithium vanadiumphosphate has good performance at low temperatures, but low specificcapacity. As such, there is a composite cathode material including bothlithium iron phosphate and lithium vanadium phosphate provided in,“Improving electrochemical properties of lithium iron phosphate byaddition of vanadium,” Yang M R, Ke W, Wu S H. J Power Sources, 2007,165: 646-650. However, in the composite cathode material, lithium ironphosphate and lithium vanadium phosphate are disorderly disposed, whichmeans that some of the lithium iron phosphate cannot contact with theelectrolyte when used in a lithium battery. As such, lithium ironphosphate cannot be dispersed in the electrolyte easily and quickly,thereby negatively impacting electrochemical properties of the electrodematerial of the lithium battery.

What is needed, therefore, is a lithium battery cathode compositematerial and method for making the same that can overcome theabove-described shortcomings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a schematic view of a lithium battery cathode compositematerial structure according to a first embodiment.

FIG. 2 is a Scanning Electron Microscope (SEM) Image of the lithiumbattery cathode composite material in FIG. 1.

FIG. 3 is a schematic view of a lithium battery cathode compositematerial doped with carbon structure according to a second embodiment.

FIG. 4 is a test graph showing charge/discharge specific capacities at0.1 Coulomb (C) rates of the lithium battery cathode composite materialaccording to the first embodiment and a lithium battery cathodecomposite material doped with vanadium according to a third embodiment.

FIG. 5 is a test graph showing charge/discharge specific capacities atdifferent rates of the lithium battery cathode composite materialaccording to the first embodiment and the lithium battery cathodecomposite material doped with vanadium according to the thirdembodiment.

FIG. 6 is a flow chart of a method for making the lithium batterycathode composite material according to one embodiment.

FIG. 7 is an SEM image of a lithium vanadium phosphate materialaccording to one embodiment.

FIG. 8 is an SEM image of lithium iron phosphate precursor particlesaccording to one embodiment.

FIG. 9 is a Transmission Electron Microscope (TEM) image of lithium ironphosphate precursor particles according to one embodiment.

FIG. 10 is an SEM image of lithium iron phosphate particles according toone embodiment.

FIG. 11 is a test graph showing the relationship between specificcapacity and cycling capability at 0.1 C rate of the lithium batterycathode composite material according to another embodiment.

FIG. 12 is a chart comparing discharging specific capacity at 0.1 Cbetween the lithium iron phosphate particles in FIG. 10 and the lithiumbattery cathode composite material according to one embodiment.

FIG. 13 is a chart comparing discharging specific capacity at 1 Cbetween the lithium iron phosphate particles in FIG. 10 and the lithiumbattery cathode composite material in FIG. 12.

FIG. 14 is a chart comparing discharging specific capacity at 5 Cbetween the lithium iron phosphate particles in FIG. 10 and the lithiumbattery cathode composite material in FIG. 12.

FIG. 15 is a chart comparing discharging specific capacity at 10 Cbetween the lithium iron phosphate particles in FIG. 10 and the lithiumbattery cathode composite material in FIG. 12.

FIG. 16 is an SEM image of lithium iron phosphate precursor particleswithout vanadium doping according to one embodiment.

FIG. 17 is an SEM image of lithium iron phosphate precursor particlesdoped with vanadium according to another embodiment.

FIG. 18 is an SEM image of lithium iron phosphate particles doped withvanadium according to yet another embodiment.

FIG. 19 is s a test graph showing the relationship between specificcapacity and cycling capability at 1 C rate of the lithium ironphosphate particles doped with Vanadium according to another embodiment.

FIG. 20 is a chart comparing X-ray diffraction patterns between thelithium iron phosphate particles in FIG. 16 and lithium iron materialsdoped with vanadium.

FIG. 21 is a chart comparing discharging specific capacity at 0.1 Cbetween lithium iron phosphate particles doped with vanadium and lithiumbattery cathode composite material doped with vanadium according to oneembodiment.

FIG. 22 is a chart comparing discharging specific capacity at 1 Cbetween lithium iron phosphate particles doped with vanadium and lithiumbattery cathode composite material doped with vanadium in FIG. 19.

FIG. 23 is a chart comparing discharging specific capacity at 5 Cbetween lithium iron phosphate particles doped with vanadium and lithiumbattery cathode composite material doped with vanadium in FIG. 19.

FIG. 24 is a chart comparing discharging specific capacity at 10 Cbetween lithium iron phosphate particles doped with vanadium and lithiumbattery cathode composite material doped with vanadium in FIG. 19.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

Referring to FIGS. 1 and 2, a lithium battery cathode composite material10 according to a first embodiment is shown. The lithium battery cathodecomposite material 10 includes a plurality of composite particles 100.The composite particles 100 have core-shell structures. Each compositeparticle 100 includes one lithium vanadium phosphate particle 102 and alithium iron phosphate layer 104 covering the lithium vanadium phosphateparticle 102. The lithium vanadium phosphate particle 102 can beconsidered a “core,” and the lithium iron phosphate layer 104 can beconsidered a “shell.”

The lithium vanadium phosphate particle 102 can be ball-shaped or almostball-shaped. A diameter of the lithium vanadium phosphate particle 102is in a range from about 1 micrometer to about 50 micrometers. In otherembodiments, the diameter of the lithium vanadium phosphate particle 102is in a range from about 5 micrometers to about 20 micrometers. In theembodiment according to FIGS. 1 and 2, the diameter of the lithiumvanadium phosphate particle 102 is about 10 micrometers.

A weight ratio between the lithium iron phosphate layer 104 and thelithium vanadium phosphate particle 102 is larger than 1.5. The lithiumiron phosphate layer 104 includes a plurality of lithium iron phosphateparticles 1042. The plurality of lithium iron phosphate particles 1042is disposed on an outer surface of the lithium vanadium phosphateparticle 102. Each of the lithium iron phosphate particles 1042 can beball-shaped or almost ball-shaped. A diameter of each lithium ironphosphate particle 1042 can be in a range from about 50 nanometers toabout 10 micrometers. In other embodiments, the diameter of the lithiumiron phosphate particles 1042 can be in a range from about 100nanometers to about 500 nanometers. The diameter of the lithium ironparticles 1042 cannot be too large in the instance the lithium ironparticles 1042 fall off the lithium vanadium particle 102. In theembodiment according to FIGS. 1 and 2, the diameter of the lithium ironphosphate particles 1042 is in a range from about 100 nanometers toabout 200 nanometers. The lithium iron phosphate layer 104 has a largespecific surface area because the lithium iron phosphate particles 1042are ball-shaped or almost ball-shaped and have small diameter.

A thickness of the lithium iron phosphate layer 104 can be less than 10micrometers. In one embodiment according to FIGS. 1 and 2, the thicknessof the lithium iron phosphate layer 104 is about 2 micrometers. Thelithium iron phosphate layer 104 further includes a plurality of poresdefined by adjacent lithium iron phosphate particles 1042.

In the composite particles 100 having core-shell structures, because the“shell” (lithium iron phosphate layer) 104 has a large specific surfacearea, the “shell” 104 has a large contact area with an electrolyte whenthe lithium battery cathode composite material 10 is used in a battery,and the lithium iron phosphate particles 1042 can be dispersed easilyand quickly in the electrolyte. Further, the shell 104 includes aplurality of pores which can ensure the contact area between thecomposite particles 100 and the electrolyte is sufficient, and the“core” (lithium vanadium phosphate particle) 102 can also contact theelectrolyte. As such, the lithium ions in the lithium battery cathodecomposite material 10 can be fully dispersed in the electrolyte.Furthermore, the core 102 can be considered an active supporter of theshell 104, which further ensures the lithium iron phosphate particles1042 inside the shell 104 also disperse in the electrolyte. As such, thelithium battery cathode composite material 10 can be well dispersed inthe electrolyte.

Referring to FIG. 3, a lithium battery cathode composite material 20according to a second embodiment is shown. The lithium battery cathodecomposite material 20 includes a plurality of composite particles 200.The composite particles 200 have core-shell structures. Every compositeparticle 200 includes one lithium vanadium phosphate particle 202 and alithium iron phosphate layer 204 covering the lithium vanadium phosphateparticle 202. The lithium iron phosphate layer 204 includes a pluralityof lithium iron phosphate particles 2042.

Each of the composite particles 200 includes carbon material. The carbonmaterial in the composite particles 200 can exist as a carbon layer orcarbon particles. A weight ratio between the carbon material in eachcomposite particle 200 and the lithium vanadium phosphate particle 202can be in a range from about 0.005 to about 0.1, particularly, theweight ratio between the carbon material and the lithium vanadiumphosphate particle 202 can be in a range from about 0.02 to about 0.05.

In one example, the composite particle 200 can include a carbon layer208 disposed between the lithium vanadium phosphate particle 202 and thelithium iron phosphate layer 204. The carbon layer 208 is disposed on anouter surface of the lithium vanadium phosphate particle 202. The carbonlayer 208 can improve the conductivity of the composite particle 200,and further improve the conductivity of the lithium battery cathodecomposite material 20.

In another example, the lithium iron phosphate layer 204 can furtherinclude a plurality of carbon particles 206 dispersed in the lithiumiron phosphate particles 2042. Each of the carbon particles 206 can bedisposed between adjacent lithium iron phosphate particles 2042. Thecarbon particles 206 can improve the conductivity of the lithium batterycathode composite material 20. The carbon particles 206 can improve theconductivity of the lithium iron phosphate layer 204, and furtherimprove the conductivity of the lithium battery cathode compositematerial 20.

In yet another example, each of the lithium iron particle 2042 canfurther include a carbon layer 2046 disposed on an outer surface of thelithium iron particle 2042. The carbon layer 2046 can improve theconductivity of lithium iron phosphate layer 204, and further improvethe conductivity of the lithium battery cathode composite material 20.

A lithium battery cathode composite material according to a thirdembodiment is disclosed. The lithium battery cathode composite materialhas the same structure as the lithium battery cathode composite material10 disclosed in the first embodiment except for certain characteristicsof the lithium iron phosphate layer.

In the third embodiment, the lithium iron phosphate layer of thecomposite particle can be doped with different metal ions instead ofiron ions (Fe²⁺) in the lithium iron phosphate particles. The metal ionscan be nickel ions (Ni²⁺), cobalt ions (Co³⁺), magnesium irons (Mg²⁺),or vanadium ions (V³⁺). The chemical formula of the lithium ironphosphate doped with the metal ions can be LiFe_((1-xy/2))M_(x)PO₄,wherein M is the metal doping material in the lithium iron phosphate, Xis the number of M ions per one lithium iron phosphate molecule, Y ischarge number of one M ion. X can be in a range from about 0.01 to about0.08. Doping with the metal ions can decrease the chemical bonding forceof Li—O in lithium iron phosphate, and the lithium ions can be dispersedeasily in the electrolyte. As such, the rate capability and the cyclingcapability of the lithium battery cathode composite material 20 can beimproved. In the third embodiment, the metal doping material in thelithium iron phosphate is vanadium, and the chemical formula of thelithium iron phosphate doped with the vanadium ions isLiFe_(0.97)V_(0.03)PO₄. FIG. 4 shows that, at the same rate, thecharge/discharge specific capacity of the lithium battery cathodecomposite material according to the third embodiment is better than thelithium battery cathode composite material without doping. FIG. 5 showsthat, at different rates, the charge/discharge specific capacity of thelithium battery cathode composite material according to the thirdembodiment is better than the lithium battery cathode composite materialwithout doping.

A method for making a lithium battery cathode composite materialaccording to one embodiment is provided. The method includes thefollowing steps:

Step one, providing a plurality of lithium vanadium phosphate particles;and

Step two, forming a lithium iron phosphate layer on an outer surface ofeach lithium vanadium phosphate particle.

Step one includes the following substeps of:

S1: providing a solution including a lithium source material, a vanadiummaterial, a phosphate source material solution, and a solvent, whereinthe lithium source material, the vanadium material, and the phosphatesource material solution are dispersed in the solvent;

S2, dispersing a carbon source compound into the solution to form a solmixture;

S3, spray drying the sol mixture to form a plurality of lithium vanadiumprecursor particles; and

S4, heating the precursor particles, thereby forming a plurality oflithium vanadium phosphate particles.

In step S1, a mol ratio between lithium, vanadium, and phosphate can beabout α:2:3, and α is in a range from 3 to 3.3. The vanadium in thesolution is vanadium ions (V⁵⁺). The lithium source material and thephosphate source material can be dissolved in water. The lithium sourcematerial can be lithium hydroxide or lithium salt. The lithium salt canbe lithium carbonate, lithium sulfate, lithium nitrate, or lithiumchlorinate. The phosphate source material can be phosphateic acid,ammonium di-hydrogen phosphate, or DAP. The vanadium source material canbe ammonium metavanadate, vanadium pentoxide, hypovanadic oxide, orvanadium tetrachloride. The solvent can be water, ethanol, or acetone.The water can be deionized water or distilled water. In one embodiment,the lithium source material is lithium hydroxide, the vanadium sourcematerial is ammonium metavanadate, and the phosphate source material isphosphateic acid. The mol ratio between lithium, vanadium, and phosphatecan be about 3:2:3. The solution can be stirred about 2 hours to ensurethe lithium source, the vanadium source material, and the phosphatesource material are uniformly dissolved in the solvent.

In step S2, the carbon source compound is organic material, which canundergo schizolysis to form carbon material, such as saccharose,dextrose, phenolic resin, polyacrylic acid, polyacrylonitrile,polyethyleneglycol, and polyvinylalcohol. The carbon source compound canbe a reductive agent, and V⁵⁺ in the solution is reduced to V³⁺. A molratio between vanadium and carbon can be in a range from about 1:1 toabout 1:1.3. In order to get a uniform sol mixture, before adding thecarbon source compound or during dispersing the carbon source compound,the solution can be heated to a temperature of about 60° C. to about 85°C., resulting in partial evaporation of the solvent. In the heatingprocess, the solution can be stirred using a magnetic agitating method,a ball milling method, or ultrasonic concussion.

In one embodiment, the carbon source compound is saccharose and the molratio in the sol mixture is about 1:1.2. The detail process of the stepS2 includes:

heating the solution to about 80° C.;

maintaining the temperature and agitating the solution using themagnetic agitating method for about 2 hours until the solution becomesthe sol mixture; and

adding saccharose into the sol mixture and agitating the sol mixtureuntil the saccharose is dissolved in the sol mixture.

In step S3, the sol mixture can be dried using a spray dry method. Thespray dry method can be achieved using an airflow spray dryer. The spraydryer includes an atomizer and a peristaltic pump. The atomizer includesa two-fluid nozzle. The step S3 includes the following substeps of:

S3a, filling the sol mixture into the spray dryer using the peristalticpump;

S3b, atomizing the sol mixture using the atomizer under a certain airpressure, thereby forming a plurality of vaporific liquid drops; and

S3c, heating the liquid drops in hot air, thereby forming a plurality oflithium vanadium precursor particles.

In step s3b, the plurality of vaporific liquid drops has extremely smalldiameters. Accordingly, the liquid drops have large specific surfaceareas and rapid heat exchange can occur between the hot air and thevaporific liquid drops. Therefore, solvent in the liquid drops can berapidly vaporized, thereby forming the lithium vanadium precursorparticles. The lithium vanadium precursor particles are ball-shaped andporous. The diameters of the lithium vanadium precursor particles can bein a range from about 5 micrometers to about 20 micrometers. Each of thelithium vanadium precursor particles is a compound mixture of vanadiumsource material, lithium source material, carbon source compound, andphosphor source material.

In step S4, the lithium vanadium phosphate precursor particles areheated in an inert gas from about 10 hours to about 20 hours at atemperature ranging from about 500 degrees Celsius to about 1000 degreesCelsius. In one embodiment, the heating temperature is about 800 degreesCelsius and the heating time is about 16 hours. After the heatingprocess, the lithium vanadium precursor particles become lithiumvanadium phosphate particles. In the heating process, the carbon sourcecompound is pyrolyzed to form carbon, and V⁵⁺ ions are reduced to V³⁺ions by carbon, and carbon is oxidized to carbon dioxide or carbonmonoxide. As such, carbon may be present in the lithium vanadiumphosphate particles. Referring to FIG. 7, the lithium vanadium particlesare formed directly from the lithium vanadium precursor particles whichare ball-shaped or almost ball-shaped, and the lithium vanadiumphosphate particles are ball-shaped or almost ball-shaped.

The step two includes the following substeps of:

M1, providing a lithium iron phosphate precursor slurry;

M2, dispersing the lithium vanadium phosphate particles in the lithiumiron phosphate precursor slurry, coating the outer surface of eachlithium vanadium phosphate particle with the lithium iron phosphateprecursor slurry to form a plurality of compound precursor particles,and then solidifying the compound precursor particles; and

M3, heat treating the compound precursor particles to form a pluralityof compound particles, thereby forming the lithium battery cathodecomposite material.

In step M1, a method for making the lithium iron phosphate precursorslurry can be a coprecipitation method or a sol-gel method. In oneembodiment, the method for making the lithium iron phosphate precursorslurry includes steps of:

M1a, providing a solution including a solvent, an iron salt material,and a phosphate material;

M1b, providing a reactor, adding the solution and a plurality of microparticles into the reactor, and adding an alkaline solution into thesolution until a pH value of the solution ranges from about 1.5 to 5,and stirring the solution to form a plurality of iron phosphateprecursor particles, wherein the plurality of iron phosphate precursorparticles is disposed in the solution to form a liquid mixture; and

M1c, adding a lithium source solution and a reducing agent into theliquid mixture to form a lithium iron phosphate precursor slurry.

In step M1a, a ratio between the iron and the phosphate can be in arange from about 1:0.8 to about 1:1.2. The iron salt and the phosphatesource material can be both dissolved in the solvent to form a solution.The iron salt can be iron chloride, iron nitrate, or iron sulfate. Thephosphate source material can be phosphateic acid, ammonium hydrogenphosphate, or ammonium di-hydrogen phosphate. The phosphate sourcematerial is dissolved in the solvent to form a plurality of phosphateanions (PO₄)³⁻. The solvent can be water, ethanol, or acetone. The watercan be deionized water or distilled water. In the liquid mixture, a molratio of the iron salt is in a range from about 0.1 mol/L to about 3mol/L, and a mol ratio of the phosphate source material is in a rangefrom about 0.1 mol/L to about 3 mol/L. In one embodiment, the iron saltis iron nitrate, the phosphate source material is phosphateic acid, andthe mol ratio of the iron nitrate and the phosphateic acid are both 0.2mol/L.

In the step M1b, the plurality of micro particles is made of rigidmaterial that is not dissolved in the solvent. The micro particles donot react with the iron salt and the phosphate source material. Thematerial of the micro particles can be ceramic, quartz, or glass. Thediameters of the micro particles can be in a range from about 20micrometers to about 1 millimeter. A volume percentage of the microparticles in the solution can be in a range from about 15% to about 50%.

In the step M1b, the solution can be transported continuously in thereactor at a flow rate of about 100 ml/hour to about 150 ml/hour. In oneembodiment, the flow rate is about 120 ml/hour. The solution can beadded in the reactor from a bottom part of the reactor. A solventmaterial can be added into the reactor before the solution is added inthe reactor. The solvent material can be the same as the solvent in thesolution. In one embodiment, the solvent material is deionized water anda volume ratio of the solvent material in the reactor is about 60%.

In the step M1b, the micro particles can be added in the reactor beforethe solution is transported or during transportation of the solutioninto the reactor. The solution and the micro particles can be agitatedusing magnetic agitation, ball milling, or ultrasonic concussion. In oneembodiment, the solution and the micro particles are magneticallyagitated, the power of the magnetic agitator can be in a range fromabout 50 W/L to about 60 W/L.

In the step M1b, the alkaline solution can be ammonia water or sodiumhydroxide. In one embodiment, the pH value of the solution after thealkaline solution has been added is about 2.3. The iron salt reacts withthe phosphate source material to form a plurality of iron phosphatehydrate particles. Because the solution is added into the reactorcontinuously, the iron phosphate hydrate particles overflow from thereactor continuously. Iron phosphate hydrate particles can be collectedfrom the overflowing materials.

In step M1b, during the stirring process, the micro particles are usedas stirrers to enhance reaction between the iron salt and the phosphatesource material. The micro particles can also be used to controldiameters of the iron phosphate precursor particles, and preventing theiron phosphate precursor particles from clumping together duringformation. The micro particles can be omitted in the step.

Other parameters can be used to control diameters of the iron phosphatehydrate particles, such as the temperature of the reactor, flow speed ofthe solution, and reaction time between the iron salt and the phosphatesource material. The temperature of the reactor can be in a range fromabout 25° C. to about 50° C. The reaction time can be in a range fromabout 40 minutes to about 2 hours. The reaction time can be controlledby changing the flow velocity of the solution. The higher thetemperature, the larger the diameters of the iron phosphate particles.The longer the reaction time, the larger the diameters of the ironphosphate precursor particles. In one embodiment, the temperature of thereactor is about 25° C. and the reaction time is about 1 hour.

The step M1b can further include the step of washing the iron phosphatehydrate particles. The overflowing materials collected are washed usinga centrifugation method. The iron phosphate precursor particles aredried at a temperature of about 70° C. to about 100° C. for about 2hours to about 4 hours. The diameters of the iron phosphate hydrateparticles are in a range from about 20 nanometers to about 10micrometers. The diameters of the iron phosphate hydrate particles aresmaller than the micro particles. The iron phosphate hydrate particlesand the micro particles can be separated by filtering.

Referring to FIGS. 8 and 9, the diameters of the iron phosphate hydrateparticles can be in a range from about 20 nanometers to about 10micrometers. The shapes of the iron phosphate hydrate particles can beball-shaped or almost ball-shaped. In one embodiment, the diameters ofthe iron phosphate hydrate particles are in a range from about 100nanometers to about 200 nanometers. Characteristics such as thediameters and dispersivity of the iron phosphate hydrate particlesdetermine the characteristics of the lithium battery cathode compositematerial.

The iron phosphate hydrate particles can be further heated to atemperature of about 400° C. to about 700° C. in a protective gasatmosphere to remove the crystal water in the iron phosphate hydrateparticles, thereby forming iron phosphate precursor particles withoutcrystal water. The protective gas can be inert gases or nitrogen. Thetemperature is maintained for about 2 hours to about 24 hours. In oneembodiment, the iron phosphate hydrate particles are heated to about520° C. for about 10 hours to remove crystal water to form the ironphosphate precursor particles.

In the step M1c, the lithium source material can be lithium hydroxide orlithium salt. The lithium salt can be lithium carbonate, lithiumsulfate, lithium nitrate, or lithium chlorination. The solvent can bewater, ethanol, or acetone. The water can be deionized water ordistilled water. The reducing agent can be a carbon source compound,which is an organic material and can undergo schizolysis to form carbonmaterial, such as saccharose, dextrose, phenolic resin, polyacrylicacid, polyacrylonitrile, polyethyleneglycol, and polyvinylalcohol. A molratio between lithium in the lithium source material, phosphate in theiron phosphate precursor particles, and carbon in the carbon compoundmaterial can be in a range from about 1:1:1 to about 1.2:1:1.3. In oneembodiment, the lithium source material is lithium hydroxide, and thereducing agent is saccharose.

In the step M1c, after the lithium source material and the reducingagent are added in the liquid mixture, the liquid mixture can beagitated by a magnetic agitating method, ball milling method, orultrasonic concussion. In one embodiment, the liquid mixture is agitatedby the ball milling method for about 2 hours.

In the step M1c, the lithium iron phosphate precursor slurry is amixture including iron phosphate precursor particles, lithium sourcematerial, and reducing agent.

In the step M1, in another embodiment, after the lithium iron phosphateprecursor slurry is formed, the lithium iron phosphate precursor slurrycan be dried and then heat-treated at a temperature in a range fromabout 500° C. to about 850° for about 8 hours to about 40 hours to forma plurality of lithium iron phosphate particles. In this process, thereducing agent undergoes schizolysis to form carbon material, and theFe³⁺ ions in the iron phosphate precursor particles are reduced to Fe²⁺ions by the carbon material. The Fe²⁺ ions react with the lithium sourcematerial to form lithium iron phosphate material, a plurality of lithiumiron phosphate particles. The carbon material can control diameters ofthe lithium iron phosphate particles and prevent the lithium ironphosphate particles from clumping together. The lithium iron phosphateparticles made by the method in this embodiment can be used as a cathodematerial in a lithium battery separately.

The step M2 further includes substeps of:

M2a, dividing the lithium iron phosphate precursor slurry into a firstpart, a second part, and a third part;

M2b, dispersing the lithium vanadium phosphate particles in the firstpart, to coat the surface of each of the lithium vanadium phosphateparticles with the first part;

M2c, separating the coated lithium vanadium phosphate particles from thefirst part, and drying the coated lithium vanadium phosphate particlesto form a plurality of first compound particles;

M2d, dispersing the first compound particles in the second part, andrepeating steps M2b and M2c to form a plurality of second compoundparticles;

M2e, dispersing the second compound particles in the third part, andrepeating steps M2b and M2c to form a plurality of compound precursorparticles.

In the step M2b, the lithium vanadium phosphate particles and the firstpart lithium iron phosphate precursor slurry are agitated to ensureuniform coating of each of the lithium vanadium phosphate particles.

In the step M2b, a water-soluble adhesive can be added into the lithiumiron phosphate precursor slurry. The lithium iron phosphate precursorslurry with the water-soluble adhesive can tightly combine with thelithium vanadium phosphate particles.

In the step M2c, the lithium vanadium phosphate particles coated withthe lithium iron phosphate precursor slurry are dried at a temperaturein a range from about 60° C. to about 90° C. for about 10 minutes toabout 30 minutes.

In the step M2e, each compound precursor particle includes a lithiumvanadium phosphate particle and a lithium iron phosphate precursorlayer. The lithium iron phosphate precursor layer includes ironphosphate precursor particles, lithium source material, and reducingagent. A weight ratio between the iron phosphate precursor particles andthe lithium vanadium phosphate particles in the compound precursorparticles can be in a range from about 5.5:4 to about 6.5:4.

In the step M3, the compound precursor particles are heat-treated at atemperature in a range from about 500° C. to about 850° for about 8hours to about 40 hours. In this process, the reducing agent undergoesschizolysis to form carbon material, and the Fe³⁺ ions in the ironphosphate precursor particles are reduced to Fe²⁺ ions by the carbonmaterial. The Fe²⁺ ions react with the lithium source material to formlithium iron phosphate material, and as such a plurality of lithium ironphosphate particles is formed, whereby the compound precursor particlesbecome compound particles. Each of the compound particles includes alithium vanadium phosphate particle and a lithium iron phosphate layerdisposed on the surface of the lithium vanadium phosphate particle. Thelithium iron phosphate layer includes a plurality of lithium ironphosphate particles. The carbon material can control the diameters ofthe lithium iron phosphate particles and prevent the lithium ironphosphate particles from clumping together. The carbon material disposedon surfaces of the lithium iron phosphate particles can improve theconductivity of the lithium iron phosphate particles.

The lithium battery cathode composite material formed by this method hasa core-shell structure, because the “shell” (lithium iron phosphatelayer) has a large specific surface area, the shell has a large contactarea with an electrolyte when the lithium battery cathode compositematerial is used in a battery, and the lithium iron phosphate particlescan be dispersed easily and quickly in the electrolyte. The method formaking the lithium battery cathode composite material is anoxido-reduction method which is simple and has a short cycle. Thereducing agent is carbon compound material, which is relatively cheap,thus the method is low-cost.

The lithium iron phosphate particles disposed on the surfaces of thelithium vanadium phosphate particles can be used as the electrodematerial of a lithium battery separately. Referring to FIG. 11, at a 1Coulomb (C) rate, the lithium iron phosphate particles made by thismethod, which have diameters of about 100 nanometers to about 200nanometers, have a specific capacity of about 106.4 mAh/g in the firstcycle, and the specific capacity of the lithium battery cathodecomposite material is decreased about 95 mAh/g at the fiftieth cycle,which is down about 10% from the first cycle. Thus, the lithium ironphosphate particles having small diameters have good cycling capability.Referring to FIGS. 12-15, at a voltage of about 2.5V to about 4.3V, thelithium battery cathode composite material made by the present methodhas a higher specific capacity than the lithium iron phosphate particlesat different rates. FIGS. 12-15 shows that the lithium battery cathodecomposite material having core-shell structures have bettercharacteristics than just lithium iron phosphate particles, because thelithium iron phosphate particles in the lithium battery cathodecomposite material is essentially a “shell” having a large specificsurface area, and the lithium iron phosphate particles 1042 can bedispersed easily and quickly in the electrolyte.

A method for making a lithium battery cathode composite material dopedwith vanadium ions is provided. The method includes the following steps:

Step I, providing a plurality of lithium vanadium phosphate particles;and

Step II, forming a lithium iron phosphate layer doped with vanadium ionson an outer surface of each lithium vanadium phosphate particle.

In step I, the detailed process of providing the plurality of lithiumvanadium phosphate particles is the same as step one of the method formaking a lithium battery cathode composite material.

Step II includes the following substeps of:

N1, providing a slurry of lithium iron phosphate precursor doped withvanadium ions;

N2, dispersing the lithium vanadium phosphate particles in the lithiumiron phosphate precursor slurry, to coat the outer surface of eachlithium vanadium phosphate particle to form a plurality of compoundprecursor particles, and then solidifying the compound precursorparticles; and

N3, heat treating the compound precursor particles to form a pluralityof compound particles doped with vanadium, thereby forming the lithiumbattery cathode composite material doped with vanadium.

In step N1, a method for making the slurry of lithium iron phosphateprecursor doped with vanadium includes steps of: N1a, providing asolution including a solvent, a vanadium source material, an iron saltmaterial, and a phosphate material; N1b, providing a reactor, adding thesolution and a plurality of micro particles into the reactor, adding analkaline solution in the solution until the solution has a pH valueranging from about 1.5 to 5, and stirring the solution to form aplurality of iron phosphate precursor particles, wherein the pluralityof iron phosphate precursor particles is disposed in the solution toform a liquid mixture; and N1c, adding a lithium source solution and areducing agent into the liquid mixture to form a slurry of lithium ironphosphate precursor doped with vanadium.

In step N1a, a mol ratio between the mol sum of vanadium, iron, and thephosphate can be in a range from about 1:0.8 to about 1:1.2. Othercharacteristics of step N1a are the same as those disclosed in the stepM1a above.

In step N1b, the iron salt reacts with the phosphate source material andvanadium source material to form a plurality of iron phosphate hydrateparticles doped with vanadium. The detailed process is the same as thestep M1b disclosed herein.

In step N1c, the detailed process is the same as the step M1c disclosedherein. In the step N1c, the slurry of lithium iron phosphate precursordoped with vanadium is a mixture including iron hydrate particles dopedwith vanadium, lithium source material, and reducing agent. Referring toFIGS. 16 and 17, the iron phosphate hydrate particles doped withvanadium have smaller diameters and better dispersivity than the ironphosphate hydrate particles without vanadium.

In the step N1, after the slurry of lithium iron phosphate precursordoped with vanadium is formed, the slurry of lithium iron phosphateprecursor doped with vanadium can be dried and then heat-treated at atemperature in a range from about 500° C. to about 850° for about 8hours to about 40 hours to form a plurality of lithium iron phosphateparticles doped with vanadium. In this process, the reducing agentundergoes schizolysis to form carbon material, the Fe³⁺ ions in the ironphosphate precursor particles are reduced to Fe²⁺ ions by the carbonmaterial, and the V⁵⁺ ions are reduced to V³⁺. The Fe²⁺ ions and the V³⁺ions react with the lithium source material to form lithium ironphosphate material doped with vanadium, and as such, a plurality oflithium iron phosphate particles doped with vanadium is formed. In thepresent embodiment, the chemical formula of the lithium iron phosphateparticles doped with vanadium is LiFe_(0.97)V_(0.03)PO₄. The carbonmaterial can control diameters of the lithium iron phosphate particlesdoped with vanadium and prevent the lithium iron phosphate particlesdoped with vanadium from clumping together. Referring to FIG. 18,lithium iron phosphate particles doped with vanadium are ball-shaped oralmost ball-shaped and have small diameters. The lithium iron phosphateparticles doped with vanadium can be used as cathode material in alithium battery separately. Referring to FIG. 19, at a voltage of about2.5V to about 4.2V and at a 1 C rate, the lithium iron phosphateparticles doped with vanadium (LiFe_(0.97)V_(0.03)PO₄) has a specificcapacity of about 135.3 mAh/g at the first cycle, and a specificcapacity of about 124.4 mAh/G after 50 cycles. Therefore the lithiumiron phosphate particles doped with vanadium have a high specificcapacity and good cycling capacity.

In step N2, the detailed process is the same as the step M2 disclosedherein.

In step N3, the detailed process is the same as the step M3 disclosedherein. In step N3, each of the compound particles doped with vanadiumincludes a lithium vanadium phosphate particle and a lithium ironphosphate disposed on the surface of the lithium vanadium phosphateparticle. The lithium iron phosphate layer includes a plurality oflithium iron phosphate particles doped with vanadium. Each of thelithium iron phosphate particles is doped with vanadium. Referring toFIG. 20, the comparison of X-ray diffraction patterns between thelithium iron phosphate particles and lithium iron phosphate particlesdoped with vanadium shows that the two X-ray diffraction patterns arealmost the same, which proves that vanadium in the lithium ironphosphate particles exists as vanadium ions instead of iron ions in thelithium iron phosphate particles, and there are no impurities brought bythe vanadium in the lithium iron phosphate particles doped withvanadium.

The lithium iron phosphate particles doped with vanadium disclosed abovecan be used as an electrode material separately. Referring to FIGS.21-24, at a voltage of about 2.5V to about 4.3V, the lithium ironphosphate particles doped with vanadium have specific capacities of148.6 mAh/g at 0.1 C rate, 135.3 mAh/g at 1 C rate, 105.0 mAh/g at 5 Crate, and 74.3 mAh/g at 10 C rate. At a voltage of about 2.5V to about4.3V, the lithium battery cathode composite material doped with vanadiumhas specific capacities of 145.0 mAh/g at 0.1 C rate, 136.2 mAh/g at 1 Crate, 115.0 mAh/g at 5 C rate, and 89.0 mAh/g at 10 C rate. FIGS. 21-24show that the lithium battery cathode composite material doped withvanadium has almost the same specific capacity as the lithium ironphosphate particles doped with vanadium at lower rates (0.1 C and 1 C),but has higher specific capacity than the lithium iron phosphateparticles doped with vanadium at higher rates (5 C and 10 C).

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the disclosure. Variations may be madeto the embodiments without departing from the spirit of the disclosureas claimed. It is understood that any element of any one embodiment isconsidered to be disclosed to be incorporated with any other embodiment.The above-described embodiments illustrate the scope of the inventionbut do not restrict the scope of the disclosure.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

1. A lithium battery cathode composite material comprising a pluralityof composite particles, each of the composite particles comprising alithium vanadium phosphate particle and a lithium iron phosphate layerdisposed on a surface of the lithium vanadium phosphate particle,wherein the lithium iron phosphate layer comprises a plurality oflithium iron phosphate particles.
 2. The lithium battery cathodecomposite material of claim 1, wherein the composite particles havecore-shell structures.
 3. The lithium battery cathode composite materialof claim 1, wherein the lithium vanadium phosphate particle in eachcompound particle is substantially ball-shaped, and a diameter of thelithium vanadium phosphate particle is in a range from about 1micrometer to about 50 micrometers.
 4. The lithium battery cathodecomposite material of claim 1, wherein the lithium iron phosphate layercomprises a plurality of pores defined by the lithium iron phosphateparticles.
 5. The lithium battery cathode composite material of claim 1,wherein the lithium iron phosphate particles are substantiallyball-shaped, and diameters of the lithium vanadium phosphate particleare in a range from about 50 nanometers to about 10 micrometers.
 6. Thelithium battery cathode composite material of claim 1, wherein a weightratio between the lithium iron phosphate layer and the lithium vanadiumphosphate particle is larger than 1.5.
 7. The lithium battery cathodecomposite material of claim 1, wherein a thickness of the lithium ironphosphate layer is less than or equal to 10 micrometers.
 8. The lithiumbattery cathode composite material of claim 1, wherein the compositeparticle comprises carbon material disposed therein, and a weight ratiobetween the carbon material in each composite particle and the lithiumvanadium phosphate particle is in a range from about 0.005 to about 0.1.9. The lithium battery cathode composite material of claim 8, whereinthe carbon material exists as a carbon layer disposed between thelithium vanadium phosphate particle and the lithium iron phosphatelayer.
 10. The lithium battery cathode composite material of claim 8,wherein the carbon material exists as a carbon layer disposed on asurface of each of the lithium iron phosphate particles.
 11. The lithiumbattery cathode composite material of claim 8, wherein the carbonmaterial exists as carbon particles disposed in the lithium ironphosphate layer.
 12. The lithium battery cathode composite material ofclaim 1, wherein the lithium iron phosphate particles are doped by metalions.
 13. The lithium battery cathode composite material of claim 12,wherein the lithium iron phosphate particles comprise a plurality ofiron ions (Fe²⁺), nickel ions (Ni²⁺), cobaltco ions (Co³⁺), magnesiumirons (Mg²⁺), or vanadium ions (V³⁺).
 14. The lithium battery cathodecomposite material of claim 13, wherein a chemical formula of thelithium iron phosphate particles doped with the metal ions isLiFe_((1-xy/2))M_(x)PO₄, wherein M is a metal doped in the lithium ironphosphate particles, X is a number of M ion in one lithium ironphosphate molecule, and Y is a charge number of one M ion.
 15. Thelithium battery cathode composite material of claim 14, wherein X is ina range from about 0.01 to about 0.08.
 16. A lithium battery cathodecomposite material comprising a plurality of composite particles, eachof the composite particles having a core and a shell, wherein the corecomprises one lithium vanadium phosphate particle, and the shellcomprises a lithium iron phosphate layer defining a plurality of pores.17. The lithium battery cathode composite material of claim 16, whereinthe lithium iron phosphate layer comprises a plurality of lithium ironphosphate particles dispersed uniformly on a surface of the lithiumvanadium phosphate particle.
 18. The lithium battery cathode compositematerial of claim 17, wherein the lithium iron phosphate particles aredoped by metal ions.
 19. The lithium battery cathode composite materialof claim 18, wherein the lithium iron phosphate particles are doped byvanadium ions (V³⁺).
 20. A lithium battery cathode composite materialcomprising a plurality of composite particles, each of the compositeparticles having a core and a shell, wherein the core is a lithiumvanadium phosphate particle, the shell is a lithium iron phosphate layercomprised of a plurality of lithium iron phosphate particles, and aplurality of pores are defined by the plurality of lithium ironphosphate particles.